The invention relates to removable dental appliances consisting of cobalt-chromium cast alloys.
Non-iron metal cast alloys on cobalt-chromium base are in use for dental purposes for many years. As cobalt and chromium, they contain molybdenum as essential alloying constituents and, in some cases, also nickel. (F. R. Morral: Journal of Materials 1 (1966), pp. 384 to 412).
In the meantime, alloys on basis of about 62% Co, 30% Cr and 5% Mo were introduced for dental appliances. They have virtually completely substituted the originally used noble metal alloys. The Cr-Cr alloys have their outstanding properties, both from a clinical and from a technological point of view. A substantial advantage of the non-iron metal alloys over the noble metal alloys consists in their much lower price. Material costs have in the meantime come to the forefront of interest in the health service due to the curbing of costs ever more necessary for economical reason.
Apart from these purely economic aspects, the material properties which have to be maintained for the suitability of a metal alloy for the indication mentioned were not to be neglected.
To be regarded as a guideline for the requirements on the material properties are the recommendations of the International Organization for Standardization (ISO), because the non-iron metal cast alloys mentioned are marketed internationally. The standard concerned ISO 6871 "Dental base metal casting alloys" is at present being printed. The present draft for this ISO/DIS 6871 of 12.07.1984 includes the following recommendations already considered final and to be regarded as governing for the alloy to be used according to the invention:
1. The requirements on the chemical composition deal not only with alloys having a total content of chromium, cobalt and nickel of at least 85%, but alternatively also with alloys having a different composition, provided that they comply satisfactorily with the requirements on toxicity and corrosion resistance. As regards these requirements, it is further specified that the alloys give off neither constituents with harmful effect to the denture wearer, nor may they have visible signs of corrosion after having been subjected to an oral environment for one year. Furthermore, it is also recommended for beryllium-containing alloys that the Be content of 2% is not to be exceeded. PA0 2. For the mechanical properties, the standard confines itself to a minimum yield strength of 500N/mm.sup.2 and to a minimum elongation A.sub.5 =3% after fracture. PA0 3. Finally, castability is to be ensured.
The price of the customary cobalt-chromium cast alloys is substantially determined by the content of cobalt. This is not only attributable to its by far predominant proportion but also to its high price in comparison with the remaining elements. Added to this is the fact that very considerable speculative, even if temporary, increases in the cobalt world market price have in recent years at times been a very great burden on the material costs of cobalt-chromium alloys. It is therefore object of the present invention to develop a non-iron metal cast alloy for dental purposes which is even more favourably in price than the customary cobalt-chromium alloys. At the same time however, it had to be borne in mind that the reduction in material costs aimed for was not to be bought by cost-increasing extra expenditure, for example in material production, in preparation of the prosthetic casting or in their subsequent finishing, and that no impairment of its properties in use occurred.